pointer string not returning expected output - c

Question

#include <stdio.h>
void revstr(char str[])
{
char temp;
int size = 0;
while(*str != '\0'){
size++;
str++;
}
for (int i = 0; i < size/2; i++)
{
temp = str[i];
str[i] = str[size-1-i];
str[size-1-i] = temp;
}
for(int i = 0; i < size; i++)
{
printf("%c\n", *(str+i));
}
}
int main()
{
char str[20];
printf("enter a string: \n");
scanf("%s", &str);
revstr(str);
return 0;
}
why is my rev string not printing the reverse of the string it is printing out some garbage value.
can you point out why?

Answer 1

After this while loop
while(*str != '\0'){
size++;
str++;
}
the pointer str does not point to the beginning of the string.
Instead you could write for example
while( str[size] != '\0'){
size++;
}
Nevertheless such a function should do only one thing: to reverse a string. It is the caller of the function that decides whether to output the reversed string.
So the function can look like
char * revstr( char s[] )
{
size_t n = 0;
while ( s[n] ) ++n;
for ( size_t i = 0; i < n / 2; i++ )
{
char c = s[i];
s[i] = s[n - i - 1];
s[n - i - 1] = c;
}
return s;
}
and in main you could write
puts( revstr( str ) );
Here is a demonstrative program.
#include <stdio.h>
char * revstr( char s[] )
{
size_t n = 0;
while ( s[n] ) ++n;
for ( size_t i = 0; i < n / 2; i++ )
{
char c = s[i];
s[i] = s[n - i - 1];
s[n - i - 1] = c;
}
return s;
}
int main(void)
{
char s[] = "Hello";
puts( s );
puts( revstr( s ) );
return 0;
}
The program output is
Hello
olleH
Of course instead of the while loop you could use the standard string function strlen.
size_t n = strlen( s );
Pay attention to that in the call of scanf
scanf("%s", &str);
the second argument shall be the expression str
scanf("%s", str);

Answer 2

I would implement it another way:
char *reverse(char *str)
{
char *wrk = str, *end = str;
if(str && *str)
{
while(*(end + 1)) end++;
while(end > wrk)
{
char tmp = *end;
*end-- = *wrk;
*wrk++ = tmp;
}
}
return str;
}
It is good to check if the parameter is correct.
I would return the value. It allows you to use the reversed string in expressions and as a function parameter. Example below:
int main(void)
{
char s[] = "Hello world";
printf("%s\n", reverse(s));
}
https://godbolt.org/z/fbo4nsn38

Answer 3

In your code you advance the pointer str to calculate its length but you forgot to reset it to the start after that. Also, your call to scanf should pass str, not its address. Moreover scanf will write beyond the end of str if the user enters a string longer than 19 characters. Try this instead:
#include <stdio.h>
#include <string.h>
void revstr(char str[])
{
char temp;
int size = strlen(str);
for (int i = 0; i < size / 2; i++) {
temp = str[i];
str[i] = str[size - 1 - i];
str[size - 1 - i] = temp;
}
}
int main()
{
char str[20];
printf("enter a string: \n");
fgets(str, sizeof(str), stdin);
str[strcspn(str, "\n")] = '\0';
revstr(str);
printf("%s\n", str);
return 0;
}